Method for improving an optical imaging property of a projection objective of a microlithographic projection exposure apparatus

ABSTRACT

A method is disclosed for improving an optical imaging property, for example spherical aberration or the focal length, of a projection objective of a microlithographic projection exposure apparatus. First, an immersion liquid is introduced into an interspace between a photosensitive surface and an end face of the projection objective. Then an imaging property of the projection objective is determined, for example using an interferometer or a CCD sensor arranged in an image plane of the projection objective. This imaging property is compared with a target imaging property. Finally, the temperature of the immersion liquid is changed until the determined imaging property is as close as possible to the target imaging property.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of international applicationPCT/EP2003/001564, filed Feb. 17, 2003, which claims priority of Germanpatent application DE 102 57 766.8, filed Dec. 10, 2002. The fulldisclosure of both earlier applications is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention generally relates to microlithographic projection exposureapparatuses as are used in the manufacture of integrated circuits andother microstructured devices. More particularly, the invention relatesto a method for improving an optical imaging property, for examplespherical aberration or the focal length, of a projection objective ofsuch an apparatus. The invention further relates to a microlithographicprojection exposure apparatus as such.

2. Description of Related Art

It is known to change the spatial position of individual opticalcomponents, for example by using manipulators, in a projection objectiveof a microlithographic projection exposure apparatus in order to improvethe imaging properties of the objective. The positional change of therelevant optical components is in this case carried out on theprojection objective once it has finally been installed, and in generalbefore it has yet been put into operation for the first time. This typeof fine adjustment may nevertheless be carried out at a later time, forexample in order to compensate for deteriorations of the imagingproperties due to ageing. A procedure often adopted in these methods isto record one or more imaging properties of the projection objective byusing a sensor arranged in its image plane. The way in which positionalchanges of individual optical components affect the imaging propertiesis then observed. The optical imaging properties of the projectionobjective can thus be optimised by adjusting the optical components.

From EP 0 023 231 B1 a microlithographic projection exposure apparatusis known that has, in order to hold a support for a semiconductor waferto be exposed, an open-topped container whose upper edge is higher thanthe lower delimiting surface of the projection objective. The containeris provided with feed and discharge lines for an immersion liquid, whichis circulated in a liquid circuit. When the projection exposureapparatus is in operation, the immersion liquid fills the interspacewhich is left between the semiconductor wafer to be exposed and aboundary surface of the projection exposure objective, which faces it.The resolving power of the projection objective is increased because therefractive index of the immersion liquid is higher than that of air.

The known projection exposure apparatus furthermore has a device forregulating the temperature of the immersion liquid, which is arranged inthe liquid circuit. The temperature of the semiconductor wafer to beexposed can thereby be kept constant, so as to avoid imaging errors dueto thermally induced movements of the semiconductor wafer.

The use of immersion liquids in microlithographic projection exposureapparatus is also known from JP 10-303 114 A. This addresses the problemthat undesirable temperature fluctuations of the immersion liquid canalso cause a deterioration of the imaging properties of the projectionobjective. The reason for this involves the dependency of the refractiveindex of the immersion liquid on the temperature. In order to resolvethis problem, various measures are proposed by which the temperature ofthe immersion liquid can be kept constant within narrow limits duringoperation of the projection exposure apparatus.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method and amicrolithographic projection exposure apparatus of the type, in whichthe optical imaging properties of a projection objective can be improvedeven more easily and effectively once it has finally been installed.

This object is achieved, with a projection objective which is part of amicrolithographic projection exposure apparatus for projecting areticle, arranged in an object plane of the projection objective, onto aphotosensitive surface that is arranged in an image plane of theprojection objective, by a method comprising the following steps:

a) introducing an immersion liquid into an interspace between thephotosensitive surface and an end face of the projection objective,which faces the photosensitive surface;

b) determining an imaging property of the projection objective;

c) comparing the imaging property determined in step b) with a targetimaging property;

d) changing the temperature of the immersion liquid until the imagingproperty as determined in step b) is as close as possible to the targetimaging property.

In order to improve the optical imaging properties of the projectionobjective, use is made of the discovery that the immersion liquidconstitutes an optical component of the projection exposure apparatus,which in principle influences its optical properties just as much as,for instance, the lenses arranged in the projection objective. Insteadof now adjusting (exclusively) the lenses or other optical componentsinside the projection objective mechanically in the optical path of theprojection objective, the invention exploits the possibility ofinfluencing the refractive index of the “immersion liquid” opticalcomponent via its temperature.

Although in principle it is also possible to change the refractive indexof the optical components contained in the projection objective via thetemperature, a temperature change is very much more difficult to achievethere since the materials used for lenses and the like have a lowthermal conductivity, which makes it considerably harder to set ahomogeneous temperature distribution throughout the optically activevolume. Conversely, the temperature of the immersion liquid can quiteeasily be brought to a predeterminable value and kept constant over thecorresponding optically active region, for example by circulating theliquid.

Since the refractive index of many liquids suitable as an immersionliquid depends only very weakly and—within small temperatureintervals—approximately linearly on the temperature, the refractiveindex of the immersion liquid can be set very precisely via thetemperature. With a projection exposure apparatus designed for awavelength of 193 nm, for example, in which the interspace between thephotosensitive surface and the end face of the projection objective isfilled with a 1 mm thick layer of water, the refractive index n=1.45 canbe varied by one hundredth of a part per thousand by raising or loweringthe temperature by 50 mK.

In order to determine the imaging property, it is in theory possible toposition an additional optical system in the image plane of theprojection objective, so that an image generated by the projectionobjective can be observed directly on a screen or through an eyepiece.More simply, the imaging property may be determined by projecting a testreticle through the projection objective and the immersion liquid onto aphotosensitive element arranged in the image plane. The imagingproperties can then be determined reproducibly and quantifiably by usinginstruments, which are known per se, to analyse the image stored on thephotosensitive element. For example, a photoemulsion may be envisaged asthe photosensitive element.

It is nevertheless particularly advantageous for the photosensitiveelement to be a sensor device, in particular a CCD sensor. In this way,the image generated in the image plane can be directly recorded andevaluated, i.e. without developing a photoemulsion or the like, in orderto determine the imaging properties.

As an alternative to this, the imaging property may also be determinedby using an interferometer as known, for example, from WO 01/632 33 A1.

By using the method according to the invention, it is possible toimprove all optical imaging properties of the projection objective whichcan be influenced by the immersion liquid. For example, the opticalimaging property to be improved may be a spherical aberration caused bythe projection objective. Such spherical aberrations occur particularlyin projection objectives with a high numerical aperture.

The optical imaging property to be improved may also, for example, bethe focal length of the projection objective. Since very accuratearrangement of the reticle in the focal plane of the projectionobjective is necessary so that a high-resolution image of the structuresto be projected—which are contained in this reticle—can be formed on thephotosensitive surface, conventional types of projection exposureapparatus often have an adjustment feature with which the support of thephotosensitive surface can be moved along the optical axis of theprojection objective. In this way, it is possible to position thephotosensitive surface in the focal plane of the projection objective.These mechanical adjustment devices, however, have quite elaboratedesigns. By changing the temperature of the immersion liquid, it is veryeasy to influence the focal length of the projection objective so thatan adjustment feature for the support of the photosensitive surface maybe obviated.

If a projection exposure apparatus comprises a sensor device—inparticular a CCD sensor—which can be arranged in the image plane of theprojection objective, a temperature control device for setting a targettemperature of the immersion liquid, and a computer unit which isconnected to the temperature control device, the latter may be used todetermine the target temperature of the immersion liquid from signalsgenerated by the sensor device.

Such a projection exposure apparatus allows automated improvement of theoptical imaging properties of the projection objective by changing thetemperature of the immersion liquid. The computer unit may, for example,be designed so that it determines the imaging properties of theprojection objective from the signals generated by the sensor device,and compares them with a target imaging property. In a control process,the computer unit then causes the temperature management device tochange the temperature of the immersion liquid until the imagingproperty as recorded by the sensor device is as close as possible to thetarget imaging property. Such a projection exposure apparatus allows anoperator to compensate automatically for certain deteriorations of theimaging properties of the projection objective, i.e. without drawing inspecialists, by changing the refractive index of the immersion liquid.Possible causes of the deteriorations may, for example, beageing-induced material modifications or fluctuations in the airpressure.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and advantages of the present invention may be morereadily understood with reference to the following detailed descriptiontaken in conjunction with the accompanying drawing in which:

FIG. 1 is a simplified representation of a microlithographic projectionexposure apparatus in a longitudinal section;

FIG. 2 shows an enlarged detail of FIG. 1, in which ray paths in theregion of an immersion liquid are indicated;

FIG. 3 shows the detail according to FIG. 2, but after raising thetemperature of the immersion liquid;

FIG. 4 shows an enlarged detail, corresponding to FIG. 2, of aprojection exposure apparatus having different imaging optics;

FIG. 5 shows the detail according to FIG. 4, but after raising thetemperature of the immersion liquid;

FIG. 6 shows a simplified representation of another microlithographicprojection exposure apparatus, having a sensor device arranged in theimage plane, in a longitudinal section.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a microlithographic projection exposure apparatus, denotedin its entirety by 10, in a longitudinal section. The projectionexposure apparatus 10 comprises an illumination system 12 for generatinga projection light beam 13, which includes a projection light source 14,illumination optics indicated by 16 and a diaphragm 18.

The projection exposure apparatus 10 furthermore comprises a projectionobjective 20 which projects a reduced image of a mask 24, arranged inits object plane 22, onto a photosensitive surface 26 arranged in animage plane 28 of the projection objective 20. The projection objective20 contains a multiplicity of optical components, only some of which(not dealt with in detail) are represented by way of example in FIG. 1.

The photosensitive surface 26 may, for example, be a photoresist whichis applied on a support 30, for example a silicon wafer. The support isfastened on the bottom of an open-topped container 32 with the shape ofa trough, which can be displaced parallel to the image plane by a firstdisplacement device denoted by 36. The container 32 is filledsufficiently high with an immersion liquid 38 so that an interspace 40between the photosensitive surface 26 and an end face 42 of theprojection objective 20 is filled completely with the immersion liquid38.

The container 32 also contains a temperature regulating device 44, whichmay include a heating device or alternatively a combined heating/coolingdevice. A temperature sensor 46, which records the temperature of theimmersion liquid 38 very accurately, is fastened on the inside of thecontainer 32.

The temperature regulating device 44 and the temperature sensor 46 areconnected, via lines which do not impede the displacement of thecontainer 32, to a temperature controller 48 which has a slide switch 50for adjusting a setpoint temperature.

The reticle 24, whose structures are intended to be projected onto thephotosensitive surface 26, can be displaced in the object plane 22 byusing a second displacement device 52, so that images of the entirestructured region of the reticle 24 can progressively be formed on thephotosensitive surface 26.

The projection exposure apparatus 10 functions in the following way:

The projection light beam 13 generated by the illumination device 12passes through the structures of the reticle 24 and enters theprojection objective 20. On the photosensitive surface 26, the latterforms a reduced image of the structures through which the projectionlight beam 13 has passed. In order to project the entire area of thereticle 24 onto the photosensitive surface 26, the reticle 24 may beilluminated in a “step and scan” process. In this case, the entireregion of the reticle 24 is illuminated by scanning while the seconddisplacement device 52 moves the reticle 24 through the projection lightbeam delimited by the diaphragm 18. During this scanning movement, thecontainer 32 with the support 30 fastened in it is subjected to amovement (usually in the opposite direction) by using the firstdisplacement device 36, the displacement speed of which is reducedrelative to that of the reticle 24 by the reduction ratio of theprojection objective 20.

During this displacement of the container 32, the end face 42 of theprojection objective 20 is moved through the immersion liquid 38 carriedalong by the container 32, which leads to mixing of the immersion liquid38. Such mixing is desirable since the immersion liquid 38 may becomelocally heated by the projection light passing through, so that thetemperature sensor 46 might otherwise possibly not measure thetemperature actually prevailing in the interspace 40. If the mixing dueto the displacement of the container 32 is not sufficient, additionalmixing devices may of course be arranged in the container 32. It islikewise possible to fit the container 32 in a liquid circuit, as isknown per se in the prior art. The temperature regulating device 44 andthe temperature sensor 46 may then be integrated in this temperaturecircuit alongside a filter which may optionally be provided.

If it is found during a test process when adjusting the projectionobjective, or during subsequent operation by checking the wafers whichare produced, that the imaging properties of the projection objective 20do not correspond to an intended target imaging property, for examplebecause the imaging on the photosensitive surface 26 is distorted byspherical aberration, then the setpoint temperature of the temperaturecontroller 48 is changed by actuating the slide switch 50 and theexposure is repeated. Changing the temperature of the immersion liquid38 changes its refractive index. The dependency of the refractive indexon the temperature—at least in small temperature intervals—isapproximately linear for many immersion liquids 38, so that atemperature at which one or more imaging properties of the projectionobjective 20 are improved can be determined very straightforwardly forthe immersion liquid 38 in a recursive process. The further exposure ofphotosensitive surfaces 26 is then carried out at this finally adjustedtemperature of the immersion liquid 38.

The effect of the refractive index of the immersion liquid 38 on theimaging properties of the projection objective 20 will be explained inmore detail below with reference to FIGS. 2 to 5.

FIG. 2 shows an enlarged detail of FIG. 1, in which a ray path isindicated in the region of the interspace 40 between the end face 42 ofthe projection objective 20 and the photosensitive surface 26. In thisexemplary embodiment, the end face 42 of the projection objective 20includes a flush-fitted planoconvex terminating lens 52 whichconstitutes the last optical component of the projection optics (merelyindicated by 54) of the projection objective 20. For illustrativepurposes, FIG. 2 indicates a plurality of projection light rays 56, 58,60 which propagate through the preceding optical components of theprojection objective 20 towards the terminating lens 52. Therepresentation is highly schematised and not true to scale in order toshow the effect of the temperature on the imaging properties of theprojection objective 20 more clearly.

The projection objective 20 shown in FIG. 2 produces an image distortedby spherical aberration. This means that the focal lengths arerespectively different for the near-axis projection light rays 56 andfor the off-axial projection light beams 58 and 60. In FIG. 2, only thefocal plane of the near-axis projection light rays 56 lies in the planeof the photosensitive surface 26, whereas the focal planes of theoff-axial projection light rays 58 and 60 lie in the interspace 40. Thedistance of the focal planes from the photosensitive surface 26 in thiscase increases commensurately when the projection light rays 56, 58, 60pass through the terminating lens 52 further away from the optical axisdenoted by 62.

FIG. 3 shows the detail in FIG. 2 after the temperature of the immersionliquid 38 has been raised. The immersion liquid now has a higherrefractive index than in the state shown in FIG. 2. The effect of thisis that the projection light rays 56, 58, 60 are refracted more stronglyat the interface between the terminating lens 52 and the immersionliquid 38. This stronger refraction has a commensurately greater effectthe further the projection light rays are away from the optical axis 62,since the off-axial projection light rays pass through this interface ata larger angle. The result of this is that the focal length of theprojection objective 20 is extended for the off-axial rays so that, inthe ideal case with a correspondingly selected temperature, the focalplanes of all the projection light rays 56, 58, 60 coincide with theimage plane in 28 which the photosensitive surface 26 is arranged.

By changing the temperature of the immersion liquid 38, it is thuspossible to compensate retrospectively for an inherent sphericalaberration in the projection objective 20.

FIG. 4 shows an enlarged detail, corresponding to FIG. 2, of aprojection exposure apparatus having a different projection objective120. In this embodiment, parts modified with respect to FIG. 2 aredenoted by reference numerals that are increased by 100.

Unlike the projection optics 54 shown in FIGS. 2 and 3, the projectionoptics 154 of the projection objective 120 do not have an inherentspherical aberration. The projection light rays 156, 158, 160 thereforemeet at a focal point. As can be seen in FIG. 4, however, this focalpoint does not lie in the image plane 28, i.e. the projection objective120 has a focusing error. Such a focusing error could, for example, beremedied by slightly displacing the support 30 with the photosensitivesurface 26 in the direction of the optical axis 62 with the use of asuitable displacement device. Unfortunately, the accuracy required forthis can be achieved only with great technical outlay when mechanicaldisplacement devices are used.

As shown by FIG. 5, it is likewise possible to extend the focal lengthof the projection objective 120 deliberately by raising the temperatureof the immersion liquid 38. Admittedly, this does introduce a sphericalaberration (not represented in FIG. 5). Nevertheless, the effects ofsuch a spherical aberration may be so small that they are negligible inview of the optimisation of the focal length of the projectionobjective, or they can be compensated for by other measures.

FIG. 6 shows details of another exemplary embodiment with a projectionexposure apparatus denoted in its entirety by 210, in a representationsimilar to FIG. 1. Here again, parts modified with respect to theembodiment shown in FIG. 1 are provided with reference numeralsincreased by 200.

FIG. 6 shows the projection exposure apparatus 210 in an adjustmentmode, in which the support 30 is replaced by a sensor device 64. Thesensor device 64 may, for example, be a CCD sensor, as it is known perse in the art. In the adjustment mode, the photosensitive surface 66 ofthe sensor device 64 is arranged in the image plane 28 of the projectionobjective 20. In this way, the sensor device 64 records exactly the sameimage as the one to which the photosensitive surface 26 is exposedduring the normal projection mode. The projection is in this casecarried out using a special test reticle 70, which is arranged in placeof the normal reticle 24 in the object plane 22 of the projectionobjective 20.

Instead of the CCD sensor, it is also possible to use an interferometeras the sensor device in a manner which is known per se. Wavefronts inpupil planes can thereby be recorded. This is described in more detailin the aforementioned WO 01/63233 A1.

In contrast to the projection exposure apparatus 10 in FIG. 1, theprojection exposure apparatus 210 furthermore has a computer unit 68which is connected to a temperature controller 248 for the temperatureregulating device 44. In the adjustment mode, the projection exposureapparatus 210 functions in the following way:

First, an image of the structures contained on the test reticle 70 isformed on the photosensitive surface 66 of the sensor device 64 by theprojection objective 20. This image is recorded by the sensor device 64and transmitted in digital form to the computer unit 68. From the datareceived, the latter determines a setpoint temperature which is sent tothe temperature controller 248. The temperature controller 248 thenensures that the immersion liquid 38 is brought to this new setpointtemperature. The sensor device 64 records the image of the structures ofthe test reticle 70 as modified by the temperature change, and likewisefeeds these data to the computer unit 68. By using algorithms which areknown per se, the computer unit 68 establishes whether the temperaturechange has led to an improvement or deterioration in the imagingproperties of the projection objective 120. The setpoint temperature ischanged again in accordance with this result. This recursive process iscontinued until it is no longer possible to improve the imagingproperties of the projection objective by changing the temperature.

1. A method for improving an optical imaging property of a projectionobjective which is part of a microlithographic projection exposureapparatus for projecting a reticle, arranged in an object plane of theprojection objective, onto a photosensitive surface that is arranged inan image plane of the projection objective, said method comprising thefollowing steps: a) introducing an immersion liquid into an interspacebetween the photosensitive surface and an end face of the projectionobjective, which faces the photosensitive surface; b) determining animaging property of the projection objective; c) comparing the imagingproperty determined in step b) with a target imaging property; d)adjusting the imaging property by changing the temperature of theimmersion liquid until the imaging property as determined in step b) isas close as possible to the target imaging property.
 2. The method ofclaim 1, wherein the imaging property is determined by projecting a testreticle onto a photosensitive element arranged in the image plane. 3.The method of claim 2, wherein the photosensitive element is a sensordevice.
 4. The method of claim 3, wherein the sensor device is a CCDsensor.
 5. The method of claim 1, wherein the imaging property isdetermined by using an interferometer.
 6. The method of claim 1, whereinthe optical imaging property is the spherical aberration caused by theprojection objective.
 7. The method of claim 1, wherein the opticalimaging property is the focal length of the projection objective.
 8. Amicrolithographic projection exposure apparatus, comprising: a) aprojection objective having an object plane and an image plane and whichis configured for projecting a reticle arranged in the object plane ontoa photosensitive surface arranged in the image plane, b) a fillingdevice for introducing an immersion liquid into an interspace formedbetween the photosensitive surface and an end face of the projectionobjective, c) a sensor device for generating a signal indicative of animaging property of the projection objective, wherein the sensor deviceis configured to be arranged in the image plane of the projectionobjective, d) a temperature control device for setting a targettemperature of the immersion liquid, and e) a computer unit operablyconnected to the temperature control device and the sensor device, thecomputer unit adapted to determine the target temperature of theimmersion liquid from the signal generated by the sensor device.
 9. Theapparatus of claim 8, wherein the sensor device comprises a CCD sensor.10. A microlithographic projection exposure apparatus, comprising: a) aprojection objective having an object plane and an image plane and whichis configured for projecting a reticle arranged in the object plane ontoa photosensitive surface arranged in the image plane, b) an immersionliquid in an interspace between the photosensitive surface and an endface of the projection objective, said immersion liquid having atemperature that is selected such that the immersion liquid compensatesfor a measured aberration of the projection objective.
 11. The apparatusof claim 10, wherein the aberration is a spherical aberration.